1. Field of the Invention
The present invention relates to a biodegradable protein and starch based thermoplastic composition with natural cellulosic fiber. The invention can be prepared as a biodegradable low ratio expanded foam useful as a low density foam packaging material or food container.
2. Technology Background
There is a continuing need in the food packaging art to provide materials and articles that are disposable, completely biodegradable, possess good mechanical properties at a low price, and environmentally friendly.
In recent years, people have become more aware of environmental pollution caused by plastics. Efforts have been made in developing biodegradable plastics, such as protein-based plastics and starch based plastics. These biodegradable thermoplastic compositions have been developed in an attempt to replace nondegradable petroleum-based thermoplastics for manufacturing packaging material. In the 1930-1940's and 1980's, protein and starch were used as a "filler" with conventional petroleum-based thermoplastics which made it possible to decrease the raw material cost or facilitate material destruction. (Satow, Sadakichi, U.S. Pat. Nos. 1,245,975 and 1,245,976 (Nov. 6, 1917); Sturken, Oswalt, U.S. Pat. No. 2,053,850 (Sept. 8, 1936); U.S. Pat. Nos. 1,245,978, 1,245,983, and 1,245,984 (Nov. 6, 1917)).
Protein-based plastics such as protein fiber, edible film or other articles have been made by extrusion, casting, or injection molding. One such example is described in U.S. Pat. No. 5,523,293, "Soy Protein-Based Thermoplastic Composition For Preparing Molded Articles," where articles were made from a composition having soy protein as the main material, starch as a carbohydrate filler, and requiring a reducing agent for the purpose of breaking protein disulfide bonds.
A typical starch-based plastic is described in U.S. Pat. No. 4,592,795, "Non-edible, Reinforced Amylose and Amylose Starch Food Casings," which discloses food casings made of a web formed from a fibrous material such as abaca fiber, wood fiber or synthetic polymer web, and curing the impregnated web in a high amylose starch with cross-linked matrix. Another example is described in U.S. Pat. No. 4,863,655, which discloses a low density biodegradable packaging material made of high amylose starch preferably modified by propylene oxide, alkylene oxide, or polyvinyl alcohol.
A continuing problem of protein and starch based plastics is water resistance and poor aging properties. These plastics tend to absorb water over time which interacts with the protein or starch weakening its mechanical properties. In other words, they degrade too quickly. Articles made from protein or starch plastics usually have good initial mechanical properties, but do not last more than a month or two.
Other recent patents disclose starch-based compositions containing inorganic fillers. U.S. Pat. No. 5,549,859, discloses a method for the extrusion of highly plastic and moldable hydraulically settable compositions. U.S. Pat. No. 5,545,450, discloses molded articles having an inorganically filed organic polymer matrix. U.S. Pat. No. 5,660,900, discloses inorganically filled, starch-based compositions for manufacturing containers and other articles having a thermodynamically controlled cellular matrix. U.S. Pat. No. 5,618,341, discloses a method for uniformly dispersing fibers within starch-based composition. U.S. Pat. No. 5,580,624 discloses food and beverage containers made from inorganic aggregates and polysaccharide, protein, or synthetic organic binders, and the methods of manufacturing such containers. In these products, starch, protein or cellulose (cellulose derivatives) were modified as a binder or thickening agent to form a fibrous slurry composition. The calcium carbonate, hollow glass spheres, or other inorganic filler was the main ingredient with small amounts of fiber. Large amounts of inorganic material in land fills are detrimental to soil ecology.
In view of the foregoing, it would be an advancement in the art to provide biodegradable thermoplastic compositions which are sufficiently water resistant and which possess good mechanical and aging properties. It would also be an advancement in the art to provide biodegradable thermoplastic compositions which can be recycled or ingested. It would be a further advancement in the art to provide biodegradable thermoplastic compositions which enhance the soil, if disposed therein.
Such biodegradable thermoplastic compositions are disclosed and claimed herein.